The present invention is generally directed to a method for controlling access to globally available data which resides on one node of a multiprocessor system so as to maintain consistency in a distributed peer to peer data processing system group. More particularly, the present invention is directed to a method and system for maintaining consistency of data without sacrificing access optimality and a loss of efficiency.
The present invention is employed in a distributed data processing system in which there are a plurality of separate and independent processor nodes each running their own set of local processes. In the systems of the present invention one or more of these nodes includes a global resource such as configuration data which must be accessed from and by other processes running in the other nodes in a group. In particular, when the global resource is changed, consistency problems can arise. Additionally, when a processor enters or leaves the group and/or when group start-up is initiated or reconfigured, consistency problems can also arise. It is difficult not only to notify all peer processes about changes in a global resource (such as its content) in a synchronized manner such that different resource content is used among the peer processes, but it is also especially difficult to optimize access to the global resource in terms of minimizing the number of accesses to needed global data. In many cases, if a few peer processes miss the notification of the changes, or receive the notifications asynchronously as is often the case, overall coordination between peer processes may fail or may cause unexpected results.
There are several situations in which the inconsistency problem is significant. For example, a global resource may be located on a processor node which is accessed from many other nodes at the same time. As indicated above, it is not easy to achieve notification to all distributed peer processes running on multiple nodes in a synchronized manner whenever the global resource is changed. Also, if global resource utilization is not optimized and multiple peer processes start up at almost the same time, the node which has the global resource can become a performance bottleneck because all of the processes may attempt to access this resource at the same time.
Additionally, if there is no automatic mechanism which allows the peer processes to use the same global resource, there is always a chance for inconsistent use of the global resource between the peer processes. As a simple example of this situation, it may happen that if one of the peer processes does not receive notification of the change of the global resource, consistency is almost always lost. Lastly, if global resources change while a peer process is temporarily isolated, it is not easy for the peer process to obtain a current level of information relating to the global resource when isolation is removed.
In distributed processing environments of the kind considered herein, it is noted that there are a plurality of data processing nodes employed. Some of these nodes may in fact be physically and/or geographically remote. It is often noted that this environment includes local processes that are running on individual ones of the nodes and that these nodes may from time to time enter or drop out of an unspecified data processing group. Additionally, it is possible that nodes may fail and/or that communications to a particular node may be lost. In these situations, appropriate protocols and communications are of particular import when the nodes are accessing data of a global nature which is maintained as such on only one of the nodes. In order that access to the global version of the data is not being performed required at all times, the nodes individually maintain local copies of the global resource. This is particularly important in those situations in which the global resource represents configuration data for the group.
It is important for applications (processes running on local nodes) to have a mechanism for notifying all of the other peer processes on the other nodes that changes associated with the global resource have been made. This notification should be made in a synchronized manner as well as providing a mechanism for the automatic propagation of the changes. This notification is provided between the various peer processes running on the local nodes. These processes should execute in an environment in which there is an infrastructure which provides synchronization and automatic propogation of the changes of the global resource. However, it is difficult to create this infrastructure particularly if the processes are left on their own to create it. This situation could run the risk of repeating errors and increases the risk of loss of data consistency among the nodes. Accordingly, the present invention provides infrastructure support which is available to all processes running and associated with a group of nodes or processes.